Electrodeless lamps, such as those shown in U.S. Pat. No. 5,834,905, employ a hollow glass envelope containing mercury vapor and a buffer gas and having a phosphor coating on the inside surface of the glass body. The phosphor is a substantially homogeneous mixture of usually three materials having emission spectra in different parts of the visible spectrum and blended to emit white light. In preferred forms of this lamp the blend includes three phosphors, namely, red emitting Y2O3:Eu3+, green emitting (Ce, Tb)MgAl11O19:Ce3+, Tb3+ and a blue emitting selected from the group of BaMgAl10O17:Eu2+ or BaMg2Al16O27:Eu2+. These lamps are designed for extremely long life, i.e., in the neighborhood of 100,000 hours. The long life of these lamps has given rise to problems involving a color shift in the emitted spectra. It has been determined that this color shift is caused by degradation of some of the lamp phosphors during life, the degradation probably being caused by the long exposure to at least one of several wavelengths of ultraviolet radiation generated during operation of the lamp.
Plasma display devices also use similar phosphors some of which can degrade over the life of the display due to long exposure to at least one of several wavelengths of vacuum ultraviolet radiation (140–200 nm). For example, a PDP display device may use Y2O3: Eu3+ or (Y, Gd)BO3:Eu3+ red phosphors, Zn2SiO4:Mn2+ green phosphor and BaMg2Al16O27:Eu2+ blue phosphor.